Controllable output circuit



5. C. VESTAL, JR

CONTROLLABLE OUTPUT CIRCUIT Jan. 4, 1949.

3 Sheets-Sheet 1 Filed on. 8, 194a M M. n m M 5 Output C/I'CUI/ oscill for/g 2'0 INVENTOR.

= small R i Y T EDWIN C. VESTAL J/t ATTORNEY Jab. 4, 1949.

Filed Oct. 8, 1946 Fig. 4

20 large R E. c. VESTAL, JR

CONTROLLABLE OUTPUT CIRCUIT 3 Sheets-Sheet 2 INVENTOR EDWIN 0. VEST/IL Jr ATTORNEY 1949- E. c. VESTAL, JR 2,457,854

CONTROLLABLE OUTPUT CIRCUIT I Filed Oct. a, 1946 a Sheets-Shaat a Fig. 6

Z0 reaclance I INVENTOR EDW/N C. VEST/1L JI.

ATTORNEY Patented Jan. 4, 1949 UNITED STATES PATENT OFFICE (Granted under the act 01 March 3. 1883, asv amended April 30, 1928; 3'20 0. G. 757) This invention relates to controllable output circuits, and more particularly to such circuits in which an oscillating voltage of controlled magnitude is derived across an impedance in the cathode circuit of a tube.

For many purposes, for example in test os'cil lators or signal generators, it is desirable that the magnitude of the oscillating voltage appearing on the output terminals be stabilized at a given point determined by the manual setting of a dial on the oscillator panel.

To this end it is an object of this invention to provide a circuit in which the magnitude of the oscillating voltage output is stabilized at a pre-determined value notwithstanding wide fluctuation in oscillating voltage input.

It is another object to provide such a circuit as above described which is of the cathode follower type, that is to say, a circuit in which the output is derived across an impedance. in the cathode circuit of a tube.

It is another object to provide a, circuit which will accept an oscillating input signal of varying,

'or unstabilized, magnitude and will deliver to its output terminals the oscillating signal at a stable magnitude determined by an externally controllable voltage applied to the circuit.

It is another object to provide an improved and more efficient circuit for amplitude modulating an oscillating voltage applied to the circuit.

It is another object to provide a circuit as above described in which no fixed bias is required on the grid of the tube, the bias being provided by the D. C. component of voltage across the cathode impedance.

It is a further object to provide a limiting circuit efiectiveto amplitude-limit an input signal on both positive and negative excursions, said limiting being maintained symmetrical for all limiting points.

In accomplishing these and other objects of the present invention, I have provided improved details of structure, the preferred forms of which are illustrated in the accompanying drawings, wherein:

Fig. 1 is a partially schematic diagrammatic view showing a circuit embodying the principles of this invention;

Fig, 2 is a circuit diagram of'an alternative modification which may be, used in the circuit of Fig. 1; and

- an oscillating signal from an oscillator. The

Figs. 3, 4, 5, and 6 are graphs illustrating the grid circuit is so designed that when the grid starts to go positive with respect to the cathode and thereby draws grid current, the voltage on the grid is substantially limited so that notwith standing increasing positive excursions of voltage applied to the grid circuit, the grid voltage is limited to a relatively small positive value. The oscillator output is so adjusted that although the magnitude may change greatly, the minimum magnitude of oscillation is such that upon negative excursions of input voltage the grid is always driven below plate current cut-01f, and upon positive excursions the grid is always driven above the grid conduction point described above. In this way, the magnitude of the plate current oscillations resulting from the input signal on the grid is substantially limited on both positive and negative excursions. Under this condition the magnitude of the oscillating-output signal appearing across the cathode impedance is a function solely of the plate voltage applied to the tube, and is substantially independent of the input voltage applied by the oscillator.

Variation of plate voltage on the tube not only shifts the operating characteristic but also changes the grid bias by virtue of the variation in D. 0. component of current flowing through the cathode impedance. By selecting an im pedance having a proper resistive component, the grid may not only be originally biased to the midpoint between plate current cut of! and grid conduction, but-may be continuously maintained at this midpoint for all values of control, or modulating, potential applied to the plate, this produces complete symmetry in the limiting action of the circuit upon the input signal, thereby minimizing distortion due to limiting. The proper value for the resistive component of the cathode impedance has been found to be equal to the reciprocal of the. mutual conductance of the tube, 1. e., l/Gm.

Referring more in detail to the drawings:

In Fig. 1, the circuit is shown comprising a tube Ill having a plate H, a grid l2 and a cathode l3. An impedance I4 is connected to cathode l3 in typical cathode follower style, the other end being grounded at l5.- Energy is supplied to tube It from a circuit l6 connected'to plate ll through isolating inductance l'l. Capacitor i8 cooperates with inductance ii to maintain plate i I at substantially zero alternating potential.

An oscillator 19 is coupled to grid l2 through a coupling coil 20, the other end of which is grounded at 2|.

Plate potential for tube I is supplied through circuit Hi from a suitable low impedance source of control, or modulating, potential 22. Interposed between source 22 and plate H are cathode follower stages 23 and 24. Cathode follower 23 has three resistors 25, 26 and 21 in its cathode circuit, the first two being variable, and the third being a potentimeter having a variable contact 28 from which output is taken.

Output circuit 29 connected across impedance l4 utilizes the oscillating signal supplied by oscillator IS, the magnitude of which is stabilized in accordance with the potential applied to plate ii.

In the circuit of Fig. 1, grid I2 is shown grounded at 2|, thereby being biased with respect to cathode l3 solely by the D. C. component of current in impedance i4.

Output coupling coil 20- is so designed that upon current flow in the circuit of grid l2, the efiective impedance introduced by coil 20 substantially limits the voltage on grid i2 notwithstanding a considerable positive increase in voltage from oscillator ID. This is effected by the fiow of grid current in coil 20 which lowers the Q of the grid circuit and in addition changes the effective coupling between oscillator is and coil 2|]. If desired, any suitable means of limiting grid voltage upon fiow of grid current may be employed, such as a high resistance in the grid circuit. In this event, oscillator voltage would be faithfully transmitted to grid i2 so long as no current flowed in the grid circuit, but upon flow of grid current the excessive voltage drop between oscillator l9 and grid [2 would substantially limit the positive voltage appearing on the grid.

With the circuit thus established, output from oscillator i9 is so adjusted that the minimum negative excursion of its output signal drives the grid below plate current cut oil, and the minimum positive excursion drives the grid into grid conduction. In this way plate current flowing from plate ii to cathode I3 through impedance I4 is limited on both the negative and positive excursions of oscillator signal voltage. On negative excursions the grid is driven below plate current out off while on positive excursions of oscillator signal plate current is substantially limited by limitation of the grid voltage itself through the impedance action of coil 20 described above.

There will be given below one analysis of the theory involved in the operation of the circuit of this invention. I do not, however, wish to be limited by the applicability or correctness of this theory, it being understood that the benefits of this invention may be fully derived by constructing a circuit in accordance with the description contained in each of the appended claims.

Fig. 3 illustrates a case in which impedance I 4 is resistive and of low value and where there is no external grid bias. To illustrate the magnitude controlling operation of the potential on plate ll, three illustrative values of plate potential, E E and Ep, have been shown in the tube characteristics plotted in Fig. 3.

The signal voltage Es applied to the grid circuit by oscillator I9 is shown at 30. Since no external bias for grid i2 is employed, i. e., Ec=0, the operating point, or effective bias on grid l2, will be determined by the D. C, component of voltage across impedance l4. The operating bias for grid l2 will thus be determined by the intersection of dotted line 3| and curves 32', 32" and 32", representing respectively the increasing plate potentials E E and E The steepness of dotted line 3! may be readily shown to be an inverse function of the magnitude of impedance l4.

During the first time interval 34 it will be assumed that the plate potential is high, E that is, the operating characteristic will be 32". The D. C. current in impedance I4 resulting from the application of E to plate II produces a bias as shown at 35, determined by the intersection 31 of line 3| and curve 32".

With application of an input signal Es from oscillator I9 the grid will first swing positive but will be limited at 38 where the grid starts to go positive and draw grid current, thereby loading coil 20 and limiting the grid voltage as explained hereinbefore.

By the time the grid has begun to go positive with respect to the cathode. the voltage between plate and cathode (Ep) will have dropped appreciably by virtue of the large drop across the cathode impedance I 4. This shifts operation to a different curve than 32". To simplify illustration however, it has been assumed that operation continues along the curve 32, inasmuch as the principles involved are believed to be unchanged. When Es swings negative the voltage e; on grid l2 swings at least to plate current cut-off 39 as shown at ll). In Fig. 3 the signal es has been shown as the minimum signal acceptable for proper operation of the circuit.

During time interval 36 it will be assumed that the plate potential has been lowered to E thereby decreasing the rid bias to Point 4| on curve 32". Limitation of grid voltage on positive excursions of Es will occur at 42 as explained for the case of operation in interval 34. This will limit plate current as shown at 43. Negative excursions will drive the grid below plate current cut-off represented by point 44 thereby limiting plate current in that direction as shown at 45.

Similar limitation occurs when plate potential is lowered to Ep', positive excursions of e: being limited by rid conduction and negative excursions driving the grid below plate current cutoff thereby limiting plate current in that direction.

From the above explanation, it will be seen that as long as the signal voltage e; is of suflicient magnitude to drive grid l2 below plate current cut-oil on negative excursions and into grid conduction on positive excursions. the magnitude of .the output signal in is dependent only on the magnitude of plate potential Ep. Thus the magnitude of the oscillating signal appearing across output impedance it, during interval lisa direct function of 311', during interval 38 of E and during interval 48 of Ep'.

The example illustrated in Fig. 3 has assumed the output, or cathode, impedance H to have a small resistive component. In Fig. 4 the resist ance of impedance I4 is assumed to be large. Reference to Fig. 4 will show that the general operation is substantially the same as for the case of Fig. 3, the-chief difference being that the plate current during quiescent period, 1. e., in the absence of input signal es, is much smaller than for the case of the low resistance illustrated in Fig. 3.

In the examples illustrated in both Figs. 3 and i the limiting of en introduced by tube I0 is unsymmetrical. That is to say. in Fig. 3 positive signal excursions are limited much more drastically than are negative excursions by virtue of the relatively small grid bias resulting from the small value of resistance used at I4. In Fig. 4 the converse is ti'ue; the negative-excursions of signal tary to the slope of the tube characteristics '48,

49 and 50 which are proportional to Gm.

Under this condition it" will be seen that the grid is always automatically biased halfway between cut-off and conduction. Thus during interval 5 l,'repres'en ting operation at Ep', the grid swings just to grid conduction and Just to cutofi. During interval 52, representing operation at Ep", the limiting' ofplate current due to grid conduction on positive excursions of es is exactly equal to the limiting of plate current due to driving the grid below the cut-off point, thereby maintaining substantial symmetry of limiting for all values or platepotential. In this example not only is the signal less distorted by virtue of the symmetry, but the minimum value below which es must not drop for proper operation of the circuit is smaller.

vModification Under certain circumstances it-may be desirable to'utilize a resistoras the cathode impedance It. Alternatively, a resonant circuit as shown in Fig. 2 having inductor 53 and capacitor 54 may be placedin the cathode circuit of tube l-O. Normally theresistance of such a circuit may be quite low, so that operation would be substantially as shown in Fig. 3. The circuit of Fig. 2 is generally suitable where output from oscillator I9 is of substantially constant frequency. With variable frequency, the reactance of circuit 53, 54 is variable, giving rise to unsatisfactory impedance matching with output circuit 29, Fig. 1.

For a constant frequency input, however, the circuit of Fig. 2 is often preferable to that of Fig. 1. In such cases an external grid bias Ec may be employed to bias the grid to the point of symmetrical signal limiting as shown in Fig. 6. As in the case of operation described for Fig. 5 the input signal, 6s, is shown as the minimum value permissible for proper limiting of the signal. Unlike the operation of Fig. 5, however, the limiting is unsymmetrical, the resulting plate current in this case resembling that illustrated in Fig. 4 where the grid bias is near cut-off.

The application of the circuits of Figs. 1 and 2 as modulating circuits is now readily evident by recognizing that since the magnitude of the'output signal is a direct function of the direct voltage on plate H, the output signal may be amplitude modulated by simply applying the modulat ing wave to plate ll, if desired through impedance matching circuits 23 and 24.

Operation A single example will now be given of the use of the circuit of Fig. 1 as a modulating circuit, with the resistance of impedance l4 being selected equal to the reciprocal of the mutual conductance of tube Hi. It will be understood that the circuit is equally applicable to output control through manual control of a more-or-less steady D. C. potential on plate ll; Such a use would be found where oscillator I9 is frequency-moduconstant control potential on plate I l the magni lated, and it is desired to strictly limit to a pre- I determined'value the magnitude of the output signal applied to circuit 29. Another such use would be as a signal generator where the output magnitude is desired to be controlled from an external knob on the panel. Said knob by controlling the DC. voltage on plate ll would control the magnitude of the output signal. It will likewise be understood that impedance I need not be resistive but may be a resonant circuit as shown in Fig. 2. Likewise, if desired, an external grid bias Ec may be combined with the grid bias produced by the D. C. component of cathode current in cathode impedance ll.

Referring now to Fig. 5, it will be assumed that the plate voltage is high, Ep'. In the absence of signal voltage es from oscillator l9, the tube will be biased midway between cut-off and grid. conduction 56 as shown at 51. With the application of signal voltage durin interval 5l,.grld I2 will be driven just to cut-oil and just to conduction as shown at 58 and 59, respectively.

Next, it-will be assumed that the plate voltage is lowered to E shifting the operating characteristic to curve 49. This will bias the grid to point 60, where curve 49 crosses dotted line 47' representing the action of the resistive component of impedance M. This point 60 is likewise midway between cut-oil" ti and grid conduction 56 by virtue of the complementary slopes of dotted line M and tube characteristics 48, 49,-and 50. Thus during interval 52 signal s will produce a plate current i which is limited in its negative excursion by cut-oil point 6| to the same.

extent that it is limited in its positive excursion by grid limiting on conduction. The limiting is thus symmetrical for all values of control, or modulating, potential Ep.

'In a circuit built according to the principles of this invention it has been found that for a tude of the output signal is maintained practically constant for all values of es up to 10 or 15 times the minimum permissible value.

While I have shown but two embodiments of my invention, it is susceptible to further modification without departing from the spirit of the invention. I do not wish, therefore, to be limited by the disclosures set forth, but only by the scope of the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

' I claim:

1. A controllable output circuit comprising a tube having a grid, a plate, and a cathode, an output impedance having a resistive component substantially equal to the reciprocal of the mutual conductance of said tube, one end of said impedance being connected to said cathode, means connected between said plate and the otherend of said impedance efiective to apply a controllable voltage to said plate, a grid circuit connected between said grid and said other end of said impedance,.means effective to apply to said grid circuit an oscillating input signal voltage having a minimum negative excursion suflicient to drive said grid below plate current cut-off and a minimum positive excursion sufiicient to drive said grid into grid conduction, an impedance in said grid circuit responsive to grid conduction current to su stantially limit the voltage applied to said amgsu grid, and an output circuit connected across said output impedance.

2. A controllable output circuit comprising a tube having a grid, a plate, and a cathode, an output impedance having a resistive component, one end oi said impedance being connected to said cathode, means connected between said plate and the other end of said impedance eflective to apply a controllable voltage to said plate, a grid circuit connected between said grid and said other end of said impedance, means effective to apply to said grid circuit an oscillating input signal voltage having a minimum negative excursion suflicient to drive said grid below plate current cut-oi! and\ a minimum positive excursion sufllcient to drive said grid into grid conduction, an impedance in said grid circuit responsive to grid conduction current to substantially limit the voltage applied to said grid, and an output circuit connected across said output impedance.

3. A controllable output circuit comprising a tube having a grid, a plate, and a cathode, an output impedance having one end connected to said cathode, means connected between said plate and the other end of said impedance effective to apply a controllable voltage to said plate, a grid circuit connected between said grid and said other end of said impedance, means effective to apply to said grid circuit an oscillating input signal voltage having a minimum negative excursion suflicient to drive .said grid below plate current cut-oil and a minimum positive excursion sumcient to drive said grid into grid conduction, an impedance-in said grid circuit responsive to grid conduction current to substantially limit the voltage applied to said grid, and an output circuit connected across said output impedance.

4. A controllable output circuit comprising a tube having a grid, a plate, and a cathode, an output impedance connected to said cathode having a resistive component substantially equal to the reciprocal of the mutual conductance of said tube, an input circuit connected to said grid, means connected to said plate effective to apply a controllable voltage thereto, means efieciive to apply to said input circuit an oscillating input signal voltage having a minimum negative excursion sufficient to drive said grid below plate current cut-oii and a minimum positive excursion sufiicient to drive said grid into grid conduction, an impedance in said input circuit responsive to grid conduction current to substantially limit the voltage applied to said rid, and an output circuit connected across said output impedance.

5. A controllable output circuit comprising a tube having a grid, a plate, and a cathode, an output impedance connected to said cathode having a resistive component, an input circuit connected to said grid, means connected to said plate effective to apply a controllable voltage thereto, means eiTective to apply to said input circuit an oscillating input signal voltage having a minimum negative excursion sufficient to drive said grid below plate current cut-off and a minimum positive excursion sufiicient to drive said grid into grid conduction, an impedance in said input circuit responsive to grid conduction current to substantially limit the voltage applied to said grid, and an output circuit connected across said output impedance.

6. A controllable output circuit comprising a tube having a grid, a plate, and a cathode, an output impedance connected to said cathode, an input circuit connected to said grid, means connected to said plate effective to apply a controllable voltage thereto, means negatively biasing said grid with respect to said cathode to a point above plate current cut-oil, means effective to apply to said input circuit an oscillating input signal voltage having a minimum negative excursion suflicient to drive said grid below plate current cut-oil and a minimum positive excursion suiiicient to drive said grid into grid conduction, an impedance in said input circuit responsive to grid conduction current to substantially limit the voltage'applied to said grid, and an output circuit connected across said output impedance.

7. A controllable output circuit comprising a tube having a grid, a plate, and a cathode, an output impedance connected to said cathode, an input circuit connected to said grid, means connected to said plate effective toapply a controllable voltage thereto, means effective to apply to said input circuit an oscillating input signal voltage having a minimum negative excursion suflicient to drive said grid below plate current cutoil and a minimum positive excursion sufiicient to drive said grid into grid conduction, an impedance in said input circuit responsive to grid conductlon current to substantially limit the voltage applied to said grid, and an output circuit connected across said output impedance.

8. A controllable output circuit comprising a tube having a grid, a plate, and a cathode, an input circuit connected to said grid, means negatively biasing said grid with respect to said cathode to a" point above grid cut-oil, means effective to apply to said input circuit an oscillating input signal voltage having a minimum negative excursion suificient to drive said grid below plate current cut-oil and a minimum positive excursion suiiicient to drive said grid into grid conduction, an impedance in said input circuit responsive to grid conduction current to substantially limit the voltage applied to said grid, means connected to said plate efiective to apply a controllable voltage thereto, and an output circuit connected between said plate and said cathode.

current cut-oil. and a minimum positive excursion sumcient to drive said grid into grid conduction, an impedance in said input circuit responsive to grid conduction current to substantially limit the voltage applied to said grid, means connected to said plate efiective to apply a controllable voltage thereto, and an output circuit connected between said plate and said cathode.

10. A controllable output circuit comprising a tube having a grid, 9, plate, and a cathode, an input circuit connected to said grid, means negatively biasing said grid with respect to said cathode to a point above grid cut-off, means effective to apply to said input circuit an oscillating input signal voltage havingia minimum negative excursion sufllcient to drive said grid below plate current cut-off and a minimum positive excursion sufiicient to drive said grid into grid conduction, an impedance in said input circuit responsive to grid conduction current to substantially limit the voltage applied to said grid, and means connected to said plate efiective to apply a controllable voltage thereto.

11. A method of deriving an output alternating signal of closely controlled'amplitude, comprising applying between grid and cathode of the tube an EDWIN C. VESTAL, JR.

0 REFERENCES arm!) The foliowing references are of record in the file of this patent:

5 UNITED STATES PATENTS Number Name Date 2,255,882 Hathaway Sept. 16, 1941 

